Many methods of welding are known in the art, each with its own advantages and disadvantages. Common welding processes include gas welding, oxyacetylene brazing and soldering, shielded metal arc welding (SMAW) or "STICK" welding, metal inert gas (MIG) or "wire feed" welding, gas tungsten arc welding (GTAW) or "TIG" welding, and plasma cutting. TIG welding is perhaps the cleanest, most precise of all hand-held welding operations. Although the method and apparatus of the present invention is preferably directed to a TIG welding process, one skilled in the art will appreciate that the present invention may have applications in many other welding operations.
A TIG welding process will now be described with reference to FIG. 1. In TIG welding, a concentrated high-temperature arc is drawn between a non-consumable tungsten electrode 10 and a workpiece 14, workpiece 14 being connected to the output of a welding power source (not shown) via a work clamp 24. Electrode 10 rests in a torch 16, the torch including a protective gas source 18, such as a cup, to direct a protective gas 20, such as argon, helium, a mixture thereof, or other inert or non-inert gasses, to a welding site 22 on workpiece 14. Torch 16 receives a flow of protective gas 20 from a gas supply (not shown). The welder strikes an arc by touching or scraping electrode 10 against workpiece 14 to close a circuit between electrode 10 and work clamp 24. As electrode 10 is drawn away from workpiece 14, an arc 12 is initiated. The welder then feeds a bare welding rod 26 to welding site 22, thereby creating a molten puddle 28. Molten puddle 28 hardens to leave a weld bead 30 joining two pieces of metal.
Numerous problems persist with this physical method of striking an arc because the tip of the tungsten electrode becomes molten from the current flowing between the electrodes and the workpiece. When the electrode is pulled back from the workpiece, the molten tungsten is left behind in the weld puddle. In certain circumstances, this contamination can cause a failure in the weld. Also, the welder must then resharpen or replace the electrode. Not only does this process inconvenience the welder, but it also wastes time and resources, which ultimately imparts a higher cost to each weld.
One known solution to the above problems has been to use a high frequency signal to initiate and maintain the arc. A high frequency signal ionizes the protective gas, allowing the welding power to jump the gap between electrode and workpiece. However, high frequency, too, has its drawbacks. The high voltage, low amperage noise from the high frequency circuitry often causes electrical interference with surrounding equipment, making its use unacceptable in certain applications. Also, the HA can be tough on TIG torches and work leads because the high voltage causes a stress to be applied to the insulation of the weld cables.
Another arc starting method which avoids the problems associated with the scratch start is the "lift" arc method. Lift arc starting involves touching the electrode to the workpiece without the necessary scraping to generate a spark. Known lift arc methods utilize a separate low-current power circuit, in addition to the power circuits already present in a welding device, to create a small monitoring voltage between the electrode and work clamp. Control circuitry monitors the voltage between the electrode and work clamp and, when a short is detected (i.e., the electrode has been touched to or brought in close proximity with the workpiece), enables the power circuit to provide an initial regulated current to warm, but not melt the electrode. When the control circuitry detects a significant torch-to-work voltage (i.e., the electrode is no longer touching or is not in close proximity to the workpiece), the control circuitry enables the power circuit to provide full user-selected welding power. However, the separate power circuit required to provide the small monitoring voltage leads to additional cost and complexity of the circuitry in the welding power source. Furthermore, existing lift arc start methods fail to reliably regulate the output current level before and after the short is detected.
Accordingly, there is a need for a method and apparatus for initiating and maintaining a welding arc that overcomes the limitations of known arc starting methods, such as the problems associated with a scratch start, the disadvantages of using a high frequency circuit, and the need for additional costly and complex circuitry. Moreover, such a method and apparatus would reliably regulate the output current both before and after a short is detected in the lift arc start.